Insight into the enormous forces at play during a 747-400 RTO

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This was posted at another forum I frequent and I thought it was superb-Written by Will Tidmarsh (a 744 2nd Officer and VATPAC founder) on the YSSY messageboard regarding a recent B744 RTO at KLAX I arrived in LAX the day after this happened, and naturally crew there with abuzz with discussion of the event, etc. By all accounts it was nothing short of a textbook Rejected Take Off by both pilots and cabin crew. Random RTO (and go-or-stop decision) events are inserted throughout our recurrent simulator checks, obviously to keep some semblance to the real thing. I was given four of them in my most recent check 2 weeks ago! As Stephen said, the amount of energy involved in some of these high speed, heavy weight RTOs can be quite enormous, and we would nearly *always* expect to blow at least one tire above 100kts. From the formula for Kinetic Energy (ie. energy of motion) given by: Kinetic Energy = 0.5 x mass (kg) x velocity-squared (km/hr) you can see that the energy required to be dissipated by the brakes as heat (with some assistance from thrust reversers and spoilers, when available) increases in proportion to the square of the velocity. Assuming we're taking off at MTOW (397 tonnes) in the 744 (normal at LAX), here are the Kinetic Energies for a few speeds.. Speed ... Energy 80kt (148.6 km/hr) 4348 megajoules 120kt (222 km/hr) 9783 megajoules (125% more energy for only 50% more speed) 146kt (270 km/hr) 14471 megajoules (48% more energy for 22% more speed).. this was our actual V1 decision speed the other night out of LAX. What does all this mean? It means that the decision to reject or to continue the takeoff becomes more and more critical at higher speeds, due to the enormous energies involved. The increasing risk of wheel fire, or tire or wheel loss obviously needs to be weighed against the severity of the fault or failure requiring the decision to be made. Indeed, in many cases, it is often safer to take an otherwise serviceable and controllable aeroplane into the air and deal with the problem there, before dumping fuel and returning. For this reason, Boeing has made 80kts a cutover point for RTOs (on the 747-400, anyway). Below 80 kts, the takeoff would normally be aborted for engine failure, any fire or fire warning, system failure, master caution, abnormally slow acceleration, takeoff config warnings, predictive windshear alerts, unusual noise or vibration, or if the aircraft is unsafe or unable to fly. Above 80kts and prior to V1, the takeoff is generally only aborted for fire/fire warning, engine failure, predictive windshear or if the aircraft becomes unsafe or unable to fly (in the Captain's opinion). Depending on the airline, the Captain will usually make the decision to either continue or reject the takeoff if a fault develops (this is why he keeps his hands on the thrust levers until V1). If he decides to stop, he calls 'Stopping' and initiates the manoeuvre by closing the thrust levers. This immediately activates the autobrakes in 'RTO' mode, and effectively dumps the full 3000psi of hydraulic pressure squarely onto the 16 wheel brakes (the nosewheel is obviously unbraked). The resulting deceleration can only be described as phenomenal, according to those who've done it! Obviously reversers and spoilers are also deployed, which take some (but only a little) load off the brakes. Interestingly, one of the most difficult RTO manoeuvres is the very low speed (<30-40kt), max thrust (for heavy weight) abort, where there can be a very significant thrust asymmetry with little or no rudder authority to counteract it, due to the low airspeed. In this scenario, closing the thrust levers immediately (and therefore stopping the thrust asymmetry) becomes paramount - being even slightly slow on this will almost surely end you up in the grass on the side of the runway! The rudder pedals provide only limited (7 degrees) nosewheel steering, and this is really all one has at their disposal under these circumstances, initially at least. It's a lengthy subject, but I hope this provides some insight! Best-Carl F. Avari-Cooper BAW0225http://online.vatsimindicators.net/980091/523.png| XP Pro SP3 | 2 x APC UPS | Coolermaster Stacker 830 SE | Gigabyte P35 DS3R | e8500 @ 4gHz | Tuniq Tower 120 | EVGA 8800GT 512MB | Creative X-Fi Fatal1ty | 2 x 1 GB Corsair XMS2 | 2 x 320GB WD Caviar RAID 0 | Corsair HX620W PS | CH Products Yoke-Pedals-Throttle Quadrant | Aerosoft 747MCP-EFIS-EICAS |

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The shocker would be comparing the kinetic engergy with the brake capacity. At 80 kts 4.4 gigajoules gotta be turned to heat to stop the beast... let's see now: 1000j/s = 1 watt so 1 gj/s = 1mW so on average you're going to produce 4.4mW of heat (not being strict with the conversion but it's continuing the idea).

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Hi Carl,What a most interesting article! I was particularly interested in the problems of stopping from low speeds with max thrust - difficulty in steering, etc.As a biologist, I can only wonder at the number of megajoules without having any idea of how many I would need to boil a kettle! ;)Cheers, Richard

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The first one gave me chills... thanks so much for the link.The problem is not unique to big iron. Last year I was on a flightline waiting for a pax when I watched a C-421 lifeguard flight abort a takeoff and taxi back to the ramp near where I was standing. Suddenly a lineman to my left side ran towards the plane signalling a stop. He saw what I missed, the left brake was on fire under a wet wing full of 100LL avgas. Thank goodness there were no non-ambulatory passengers for everyone was able to bale out and the responders were there to cool the wheel before anything major happened.

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